This invention pertains generally to the etching of copper during the fabrication of microelectronic packages and, more particularly, to copper etching compositions and various components derived therefrom.
At the present time, most chemical etching of copper on microelectronic substrates is performed with either copper or ferric chlorides, chromium salts, alkaline-ammonia, hydrogen peroxide-sulfuric acid or nitric acid compositions. Each of these compositions has certain limitations and disadvantages as described hereinbelow.
The metal etchants, in particular the chromium salts, create a deleterious environmental impact. It is also known that chromium salts are human carcinogens; therefore, their use and disposal are especially problematic.
Nitric acid, either alone or in combination with sulfuric acid or copper nitrate, has been reported by Battey (U.S. Pat. No. 4,695,348) to be useful for etching copper in circuit boards. However, nitrogous oxide gas is a byproduct of this process. Moreover, the process provides anisotropic etching only for certain orientations of copper (i.e., the top surface of the copper crystal structure must have a Miller index of (111) and (200) orientation). This requires first sputtering copper and then electrodepositing more copper. In lieu of sputtered copper, evaporative deposition or electroless deposition is also possible.
The alkaline-ammonia compositions are used commercially because they are relatively fast, have substantial copper-carrying capacity and are reasonably tolerant of some metal resists and some dry film resists. However, these same compositions have poor selectivity for copper versus other metals and alloys. Significant process control is required to achieve acceptable selectivity. It is also known that these compositions may not work well with fine line copper geometries. Furthermore, the dissolved copper is difficult to recover. Also, fumes from the ammonia composition present worker exposure concerns.
The hydrogen peroxide-sulfuric acid compositions used in copper etching processes are very clean to operate and can be recycled. However, these same compositions have relatively slow etching rates and require substantial cooling for stability control due to the autodecomposition reaction of the hydrogen peroxide. Additionally, both the performance of the process and the decomposition of the peroxide are very sensitive to trace impurities via homo- or hetero-catalysis. Stabilizers are necessary for peak performance but these are metal specific. Brasch (U.S. Pat. No. 4,378,270) teaches phenol-sulfonic acid for copper containing solutions. It is also known from Alderuccio, et al (U.S. Pat. No. 3,269,881) that these compositions are adversely affected by chloride or bromide ion at levels of 2 mg/liter, causing reduced etch rates. Elias (U.S. Pat. No. 4,130,455) teaches that the addition of sodium or potassium thiosulfate can counteract this effect.
But use of these additives does not address the basic problem of the catalytic decomposition of the peroxide discussed hereinabove. This decomposition has two important implications: firstly, the depletion of the peroxide in the etchant solution reduces the etchant rate; and secondly, there is potential for uncontrolled decomposition of large volumes of high temperature solutions, generating high concentrations of oxygen and increasing the safety risks therefrom. Because decomposition of the peroxide is accelerated at elevated temperatures, processing temperatures must be kept low. This adversely affects the rate of the etching process and exacerbates the already low copper-carrying capacity of the peroxide-sulfuric acid composition. Finally, the process generates voluminous quantities of copper sulfate, which are difficult to reclaim or dispose of. It is possible to electroplate out the copper from the copper sulfate. Typically, however, this copper does not deposit as an adherent homogeneous ingot deposit, but rather comes out as a powder which sluffs off and makes recovery difficult. Brasch (U.S. Pat. No. 4,378,270) describes the use of phosphoric acid to assist in producing smooth adherent copper deposits on a cathodic surface, but this is at the expense of a slower etchant rate.
The prior art etchants are used in processes to manufacture various types of microelectronic packages such as printed circuit boards having planar resistors and laminate chip carriers. For each of these packages, complex processes must be employed to circumvent the above-mentioned problems.
There are several problems in attempting to etch fine line (less than 0.004xe2x80x3 width) printed circuits when using classical processes and chemistry. These problems are defined as follows:
1) Fluid mechanics problemsxe2x80x94With classical print and etch processes, a photoresist is applied to the printed circuit board. Dry film photoresist materials are generally available in thicknesses of 0.001xe2x80x3 and greater, and typically are 0.0012xe2x80x3 or greater. As etching progresses through the Cu, the trough defined by the photoresist and the etched Cu becomes progressively deeper and the depth to width aspect ratio becomes progressively greater. As the aspect ratio increases, the etch uniformity decreases due to the non-uniform nature of getting fresh etchant into this trough. One solution to this problem has been to use liquid resist materials that can be applied by spray or electrodeposition methods at a reduced thickness.
2) Galvanic etchingxe2x80x94Many processors utilize a nickel/gold plated etch mask over the copper for etching. Due to the significant difference in electrode potentials between the Au and Cu metals, the copper is etched at an accelerated, almost uncontrollable rate in currently known chemistries, except possibly for ammoniacal etchants. Galvanic etch effects can be compensated for somewhat by increasing the width of the Ni/Au resist image, such that the etched cu image after galvanic etch effects is of the desired width. The down side to this technique is that it is not compatible with fine line/fine space etch requirements. This galvanic etch effect may also result in an undesirable line geometry after etching. Another potential solution would be to eliminate the Au, using only Ni as the etch mask. This solution would require an etchant that does not etch Ni, but does etch Cu. Until now, such an etchant has been unknown, except for chrome-sulfuric chemistries, which have their own problems of environmental impacts, waste treatment, and poor copper capacity.
3) Miscellaneous Au etch mask problemsxe2x80x94In addition to the galvanic etch problems, Au masking suffers from very high material costs, and inability to use for innerlayer etching due to the inability to subsequently adhere epoxy to Au during composite lamination of the innerlayers.
Subtractive etching to produce fine line circuits is especially difficult for external circuit layers on plated through-hole (PTH) printed circuit boards. Prior art technology has been unable to provide uniform line widths along with high-yielding processes. This problem is primarily due to the thick photoresist that must be used to tent PTHs on printed circuit boards. Furthermore, the thickness of the photoresist layer is related to the width of area to be etched; viz., the greater the thickness of the photoresist, the greater the spacing must be between the protected circuit lines. This relationship therefore limits the density of the circuit lines if thick photoresists are necessary.
To fabricate a photoresist circuit pattern, prior art methods initially apply a thin copper foil to a substrate and then subsequently apply, image and develop a photoresist to create a circuit pattern. The exposed, thin copper foil in the through-holes is electroless and/or electrolytically plated with copper followed by a nickel-gold etch mask. The photoresist is then stripped away and the underlying thin copper foil is fast-etched while the etch mask theoretically protects the newly formed circuit lines and through-holes. This process is very difficult to control because it requires electroplating thin patterns rather than full-panel plating. Furthermore, the etching process using either peroxide-sulfuric acid or alkaline-ammonia compositions induces galvanic etching between the nickel-gold and copper interface. This is the cause of undesirable circuit line cross-sections which, in turn, creates poor functional reliability.
The use of laminate chip carriers (LCCs) in electronic packaging is growing at an ever-increasing rate. The predominant use of LCCs is with wire bond chip attachment. The use of wire bond requires that circuits be fully encapsulated in nickel-gold or other noble metallurgy. Nickel-gold both provides a wire bondable surface and prevents copper migration shorts between circuit lines in the harsh operating environment of LCCs.
The preferred metallurgy for wire bonding is electrolytically-plated nickel-gold. Due to the dense central wiring used in LCCs, temporary commoning bars must be incorporated into the central wiring area. These temporary commoning bars are then removed from the circuit after nickel-gold plating has been performed. Prior art technology currently requires applying a new photoresist layer and etching the commoning bars through openings in the photoresist, typically with a cupric chloride etch solution. The photoresist is required in order to minimize galvanic etch undercutting when copper etching occurs in the presence of nickel-gold (or other noble metals) with the above-mentioned etchant compositionsxe2x80x94especially cupric chloride.
Since the photoresist does not completely provide protection from galvanic etching, current practice is to intentionally create a reduced opening size to compensate for the galvanic etch effects.
It is also known that application and removal of photoresist materials to and from the nickel-gold plated features can have adverse effects on wire bond yield and reliability. Hence, for wire bond LCCs it would be desirable, for the reasons discussed above, as well as for cost considerations, to have a commoning bar etchant or process that would not require a photoresist application. This can be achieved only by identifying a process that does not cause galvanic etching effects. Elimination of the galvanic etch effect will most importantly provide a superior etch profile to the fabricated circuit lines, allowing for higher quality devices.
For etching of planar resistors, an etchant must be selective to copper relative to the nickel/phosphorus (NiP) underlayer; otherwise, precise control must be maintained on the etchant chemistry and etch-process parameters, such as etch endpoint. Chromic acid compositions had been the etchants of choice due to their complete selectivity to copper, but environmental concerns have essentially precluded their use. None of the other prior art etchant compositions, including alkaline ammonia, are sufficiently selective to fabricate precision resistors. Due to this lack of selectivity, even with tight processing controls, manufacturing variability of planar resistors are typically high.
With hydrogen peroxide-sulfuric acid or alkaline-ammonium etchant compositions, there is a tendency for the copper to be etched more quickly from areas where fine lines are present than from larger areas. This results in etch mask undercutting of the fine lines while the larger areas are being cleared of copper. Furthermore, neither process is truly anisotropic. U.S. Pat. Nos. 4,497,687 and 4,545,850, both issued to N. Nelson, describe the use of nitric acid and nitric acid with sulfuric acid regenerator, respectively, to produce an anisotropic etch. But it is reported that in some instances this defect still occurs.
Etch masks can be made from various materials including plastics, ink, photoresists or a metal such as gold, solder, or a nickel-tin alloy. The mask serves the purpose of protecting the underlying material. In the case of microelectronic devices, the underlying material is typically copper. Common masks used in the prior art were solder or tin-nickel alloys. These masks effectively protected the underlying copper strata; but due to environmental concerns, they have essentially been eliminated from use. Noble metals and their alloys have alleviated the environmental problems associated with the heavy metal masks. However, in the process of etching copper using these masks, undercutting of the copper can occur at the interface of the noble metal surface and the copper surface, as described hereinabove, causing galvanic etching.
It can therefore be seen that although the process of etching copper in microelectronic packages has been performed for a lengthy period of time, many unresolved problems are still associated with the process. Some of these problems create limitations on designs of these microelectronic packages. Considering that requirements for higher circuit density, faster speed and greater reliability of these devices are constantly becoming more stringent, it is clearly understood that reduced complexity of manufacture and improved etchant performance are still required.
An etchant chemistry and associated process has now been invented to overcome all of these problems, and be able to etch fine line circuits. This etchant is not prone to the galvanic etch effects caused by Au masking, and further, does not etch Ni.
The present invention therefore provides for an etching composition useful in printed circuit board fabrication having the following independent advantages:
a) has high selectivity for copper;
b) eliminates undercutting due to galvanic etching;
c) has high copper carrying capacity;
d) remains stable during its use;
e) has wide process latitude;
f) has high etchant rates at room temperatures;
g) is environmentally acceptable; and
h) does not generate toxic fumes.
The present invention further provides for processes of manufacture and designs of microelectronic packages, in particular printed circuit boards, having the following advantages:
a) printed circuit boards formed from subtractive etching having excellent fine line electrical circuit quality (i.e., uniform line widths) via a high yielding process;
b) printed circuit board intermediates utilizing thin photoresist layers, thereby fabricating printed circuit boards possessing a high density of circuits due to improved imaging resolution;
c) printed circuit board intermediates utilizing thin layers of environmentally acceptable etch masks (i.e., elimination of the use of lead solder);
d) a printed circuit board fabrication process of reduced complexity by removing the need for hole tenting;
e) a process for fabricating a laminate chip carrier that has reduced complexity by eliminating the need for a second photoresist application to selectively etch commoning bars;
f) the fabrication of a laminate chip carrier that has improved wire bond yields and reliability due to elimination of the photoresist application and subsequent removal; and
g) a process for fabricating PCB""s having precision planar resistors.
The invention described herein is a persulfate etchant composition especially useful for dissolving a homogeneous copper during fabrication of microelectronic packages. The etchant is characterized by its ability to selectively etch copper in the presence of nickel, nickel-alloys which may be optionally overplated with noble metal alloys. Furthermore, no deleterious galvanic etching occurs in this etchant-substrate system so that substantially no accelerated etch of the copper occurs. The combination of high selectivity and lack of galvanic etch allows for a simplification of the microelectronic fabrication process and significant improvements in the design features of the microelectronic package, in particular higher density circuits. The persulfate etchant composition is stabilized with acid and phosphate salts to provide a process that is stable, fast acting, environmentally acceptable, has high capacity, and can be performed at room temperature. A preferred etchant composition is 230 gm/liter sodium persulfate, 3 volume % phosphoric acid and 0.058 molar sodium phosphate dibasic.
As discussed hereinabove, the invention involves formulating a composition for etching copper. The composition has a unique combination of etchant properties. The chemicals that comprise this formulation are readily available and inexpensive. In combination, the formulation has excellent shelf life and does not pose significant environmental or safety problems. The formulation, although designed to specifically etch homogeneous copper selectively in the presence of nickel, has wide applicability in the field of fabricating microelectronic packages as seen in the examples hereinbelow. The formulations of this invention have been unexpectedly found to eliminate galvanic etching at the interface of copper and nickel surfaces. The formulations also are selective for copper in the presence of other metal etch masks such as solder (Sn/Pb).
The long-standing problem of galvanic etching has limited the designs and capabilities of the microelectronic packages and has created additional manufacturing complexity in order to circumvent this problem. An example of one type of limitation in design is where thick, oversized, photoresist layer must be used to minimize galvanic etching when cupric chloride is used to etch commoning bars.
Another example of design limitation is when, during the standard process of subtractive etching of printed circuit boards, undercutting of the nickel/copper interface causes the resultant nickel-clad copper circuit lines to have an uneven cross-section. This defect limits the minimum size of the circuit line and, in doing so, prevents the design of more dense circuits in the printed board package.
In laminate chip carriers (LCCs) where wire bond chips are attached, the circuits in this component must be encapsulated with Ni/Au. Due to the dense central wiring used in LCCS, temporary commoning bars must be incorporated into the central wiring area. These temporary commoning bars are removed after the nickel/gold plating has been performed. Prior art practices employ an application of a new photoresist layer prior to etching the commoning bar. After etching the commoning bar with CuCl2, the photoresist is removed. The photoresist is used to minimize galvanic etch, but its application and removal deleteriously affects the wire bond yield and reliability, and adds to the manufacturing complexity.
The etchant composition of the present invention comprises a mixture of persulfate salt, a proton donor and a phosphate salt. The persulfate salts useful in this composition include alkali metals such as potassium and sodium, and quaternary nitrogen salts such as ammonium. The preferred alkali metal salt is sodium.
The proton donor materials useful in this invention include the strong and weak oxygen acids, examples of which include sulfuric acid, phosphoric acid, phosphorous acid, perchloric acid, perchlorous acid and water soluble organic acids such as acetic acid, carbonic acid and oxalic acid. The preferred acid is phosphoric acid. Hydrochloric acid and nitric acid cannot be utilized in the current invention due to toxic byproducts such as nitrogen oxides and chlorine gas. The concentration of proton donors is directly related to the well known parameter of pH by the equation pH=-log molar concentration of the proton donor, so that at pH=6 the molar proton concentration is 10xe2x88x926; while at pH=1, the molar proton concentration is 10xe2x88x921. It is seen, therefore, that minimal changes in pH actually reflect significant differences in proton donor concentration. The higher the value of the pH, the lower the concentration of the proton donor. At pH=7, neutrality is reached, wherein the proton donor solution no longer is considered to be acidic; for example, water has a pH of 7, while sulfuric acid at 0.1 molar proton concentration is pH 1. It can therefore be seen that as the pH decreases from a value of 7, there is a significantly greater safety risk due to the potential for chemical burns from strong acids. The present invention can be utilized under a wide pH range, typically from a pH of 0 to a pH of 6. Preferably, the pH of the composition should be 0.5-4 and most preferably between 1.5-3.0.
The phosphate salts useful in this composition include alkali metals such as lithium, sodium and potassium phosphate and quaternary nitrogen salts such as ammonium. Because the phosphate salt is tribasic, these salts can be either mono, dibasic or tribasic (e.g., NaH2PO4, Na2HPO4 and Na3PO4). The preferred salts are potassium and sodium phosphate dibasic. Phosphate salts have been utilized in the prior art as a protectant against corrosion; specifically with regard to steel. Phosphoric acid is also known to be added to etchant solutions on which solder (tin-lead) is employed as an etch resist. But no mention is found for the specific combination of phosphate-containing etchants in the presence of nickel. To the contrary, it has been reported that aerated phosphoric acid will corrode nickel. It is therefore an unexpected benefit that in addition to stabilizing the persulfate salt formulation, complete selectivity for etching copper in the presence of, nickel is obtained.
The initial etchant composition can comprise a mixture of 10-500 gm/liter persulfate salt, up to 50 volume % of the proton donor and up to the solubility limit of the phosphate salt. If no phosphate salt is initially included, the composition will rapidly degrade during use (e.g., during the etching process). Therefore, phosphate salt must be added during the etching process. Preferred concentrations for persulfate salts are 25-400 gm/liter and most preferably 200-300 gm/liter. The preferred concentration of proton donor is 0.5% to 25% (volume %) and most preferably at 1 to 5%. Preferred concentrations for phosphate salts are 0.01-0.2 molar and most preferably 0.03-0.10 molar. The optimal concentration for phosphate is 0.058 molar. If this is not present in the initial formulation, it should be adjusted appropriately by adding a regeneration solution of phosphate to bring the working formulation to the desired phosphate concentration. This concentration has been found to unexpectedly give complete stability to the persulfate formulation without any deleterious side effects. Adjuvants such as surfactants and banking agents can also have beneficial effects for the etchant formulation. Surfactants are incorporated into the composition primarily to reduce the adhesion of air or gas bubbles to the etched surface. If this is not provided for, there is the possibility of creating irregularly-etched surfaces due to the bubbles preventing fresh etchant from contacting the component surface. Typically, surfactants can be either anionic, cationic, nonionic and inner salt types. Anionic surfactants may contain phosphate, sulfonate or sulfate groups attached to long chain alkyl or aryl moieties.
Cationic surfactants typically are quaternary ammonium salts containing a long chain alkyl or aryl moiety. Nonionic surfactants are typically composed of long chained alkyl or aryl moieties possessing polyethylene oxy functionality. Inner salt surfactants contain both anionic and cationic functionalities that are equally balanced to create a charge-neutral material. The present invention can employ all types of surfactants described above but preferably anionic and nonionic are utilized. Most preferably anionic surfactants such as Triton X 100 are utilized.
Banking agents are employed to provide greater anisotropy during the etching process. The greater the difference between the vertical and lateral etching rates, the higher is the anisotropy of the process. An ideal etching process would be one in which the vertical etch rate is significantly higher than the lateral etch rate. High lateral etch rates cause the newly formed fine line circuit lines to have non-uniform width.
A technique to improve the anisotropy of the etch is to add materials that control the surface reaction rate typically by forming a film on the vertical walls of the newly formed fine line circuit lines. These materials, called banking agents, may be either polymeric such as polyacrylamide, or hetero aromatic organic structures such as substituted or unsubstituted benzotriazole, naphthotriazole, benzofuroxan, benzothiadiazole, benzothiazole, benzoxazole and benzimidazole. The benzene nucleus of these materials can be substituted with, for example, hydroxy, alkoxy, ammio, nitro, or alkyl groups. The preferred banking agents for this invention are benzotriazoles, the most preferred being unsubstituted benzotriazole.
The etchant process is preferably performed at room temperature, but can also be performed at higher temperatures if desired. The persulfate etchant formulation is preferably applied to the component by spraying, but immersion of the component in the formulation is feasible.
The etchant formulation is specifically designed to etch copper either in the presence of nickel or nickel alloys, or overcoated with noble metals such as porous gold and palladium. It is, however, possible to use this etchant solution with other metal masks, such as solder (Sn/Pb).